1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device in which an amorphous TFT is used in a gate line driving circuit.
2. Description of the Background Art
An active matrix liquid crystal display device includes an array substrate in which a plurality of gate lines, a plurality of source lines and a gate line driving circuit are provided, and a color filter substrate which faces the array substrate. In the color filter substrate, a coloring pattern (red, green, blue) is provided on a glass substrate. Contrast of the liquid crystal display device decreases as a result of incident light leaking from a gap between coloring layers thereof, and accordingly, a light shielding layer for shielding light is formed therebetween. This light shielding layer is also provided in a periphery area other than the display area for preventing light leakage of a backlight. This light shielding layer is made of, for example, a resin material or a metal material. A light shielding layer made of a resin is formed by mixing a light shielding material such as carbon black into a resin material, and has a relatively low light shielding rate. Accordingly, a relatively large film thickness is required for achieving sufficient light shielding property. In contrast, a light shielding layer made of a metal material generally has a high light shielding rate, and thus is capable of sufficiently exerting light shielding property with a thin film. In addition, the light shielding layer made of a resin has a structural characteristic such as being relatively vulnerable to vibration and impact. Therefore, in terms of slimming down of an entire device and its reliability, it is desired to use a light shielding layer made of a metal material, not a light shielding layer made of a resin, in a liquid crystal display device.
Further, the color filter substrate includes a counter electrode formed of an indium tin oxide (ITO) film, and this ITO film and a light shielding layer are adjacent to each other, whereby the light shielding layer is supplied with the same potential as the counter electrode in a case where the light shielding layer is made of metal. The counter electrode is generally applied with approximately 5 V.
In an active matrix liquid crystal display device in which an amorphous silicon thin film transistor (hereinafter, referred to as “TFT”) is used, as a gate line driving circuit (scanning line driving circuit) for scanning a liquid crystal display panel serving as a display part thereof, there is used a shift register which perform a shift operation in such a manner that one round is made during one frame period of a display signal. In order to reduce the number of processes in a process of manufacturing a liquid crystal display device, the shift register is desirably formed of the same conductivity type TFTs.
A liquid crystal display device in which a shift register of a gate line driving circuit is formed of TFTs is easily increased in area and has high productivity, and accordingly is used in a screen of, for example, a notebook PC, a personal digital assistant (PDA), a multimedia player (PMP) and a personal navigation device (PND) (see Jin Young Choi et al., “A Compact and Cost-efficient TFT-LCD through the Triple-Gate Pixel Structure”, Society for Information Display, 2006, International Symposium Digest of Technical Papers, pp. 274-276).
The gate line driving circuit includes a large number of contact holes for connecting different types of metal interconnections. That is, the contact holes are provided for electrically connecting a first metal thin film and a second metal thin film which are formed on different layers. The contact holes include one formed on the first metal thin film and one formed on the second metal thin film, where bridge connection is provided between both contact holes with a conductive film (contact hole part).
As a conductive material for connecting metal interconnections in which openings are provided as contact holes, a transparent conductive film such as ITO is generally used.
In order to narrow a frame of a liquid crystal display panel, it is required to, in a gate line driving circuit, dispose a sealing area which is formed so as to surround a periphery of the liquid crystal display panel and a part thereof in such a manner that they overlap each other. Generally, a portion under the sealing area is susceptible to water infiltrating from the outside, impurities or the like.
In a case of a gate line driving circuit composed of TFTs, a signal amplitude of this circuit is large such that, for example, a High voltage is 24 V and a Low voltage is −6 V, and these voltages are applied to a conductive film which provides bridge connection between the contact holes. Further, approximately 5 V is normally applied to the counter electrode as described above, whereby a large voltage of approximately +10 V is applied to the counter electrode and the light shielding layer while the Low voltage is applied to the conductive film.
In a case where a liquid crystal display panel in which a metal material is used for the light shielding layer is operated in a high temperature and high humidity environment, the liquid crystal display panel is considerably affected by water, impurities or the like as described above. When the Low voltage is applied to the conductive film, electrolysis occurs between the light shielding layer of the color filter substrate and the conductive film connected to the contact hole which is formed at a seal end (on the liquid crystal side) or under the seal. Accordingly, the light shielding layer which faces the conductive film melts into the liquid crystal layer to be lost, leading to a problem of degrading display quality, such as light leakage.